Subject: Re: [RFC PATCH net] netns: fix GFP flags in rtnl_net_notifyid()

From: Guillaume Nault <[email-masked]>
Date: Mon, 14 Oct 2019 00:22:31 +0200

On Sun, Oct 13, 2019 at 12:09:43PM -0700, Pravin Shelar wrote:
> On Thu, Oct 10, 2019 at 12:07 PM Guillaume Nault <[email-masked]> wrote:
> >
> > In rtnl_net_notifyid(), we certainly can't pass a null GFP flag to
> > rtnl_notify(). A GFP_KERNEL flag would be fine in most circumstances,
> > but there are a few paths calling rtnl_net_notifyid() from atomic
> > context or from RCU critical section. The later also precludes the use
> > of gfp_any() as it wouldn't detect the RCU case. Also, the nlmsg_new()
> > call is wrong too, as it uses GFP_KERNEL unconditionally.
> >
> > Therefore, we need to pass the GFP flags as parameter. The problem then
> > propagates recursively to the callers until the proper flags can be
> > determined. The problematic call chains are:
> >
> >  * ovs_vport_cmd_fill_info -> peernet2id_alloc -> rtnl_net_notifyid
> >
> >  * rtnl_fill_ifinfo -> rtnl_fill_link_netnsid -> peernet2id_alloc
> >  -> rtnl_net_notifyid
> >
> > For openvswitch, ovs_vport_cmd_get() and ovs_vport_cmd_dump() prevent
> > ovs_vport_cmd_fill_info() from using GFP_KERNEL. It'd be nice to move
> > the call out of the RCU critical sections, but struct vport doesn't
> > have a reference counter, so that'd probably require taking the ovs
> > lock. Also, I don't get why ovs_vport_cmd_build_info() used GFP_ATOMIC
> > in nlmsg_new(). I've changed it to GFP_KERNEL for consistency, as this
> > functions seems to be allowed to sleep (as stated in the comment, it's
> > called from a workqueue, under the protection of a mutex).
> >
> It is safe to change GFP flags to GFP_KERNEL in ovs_vport_cmd_build_info().
> The patch looks good to me.
> 
Thanks for your feedback.

The point of my RFC is to know if it's possible to avoid all these
gfp_t flags, by allowing ovs_vport_cmd_fill_info() to sleep (at least
I'd like to figure out if it's worth spending time investigating this
path).

To do so, we'd requires moving the ovs_vport_cmd_fill_info() call of
ovs_vport_cmd_{get,dump}() out of RCU critical section. Since we have
no reference counter, I believe we'd have to protect these calls with
ovs_lock() instead of RCU. Is that acceptable? If not, is there any
other way?